This invention relates to fuel and air purge systems for diaphragm carburetors adapted for use on internal combustion engines, and more particularly for use on small single cylinder four-stroke engines adapted for use on hand-held engine-driven appliances.
As a result of emission control legislation in the United States, handheld engine manufacturers have developed various solutions to the tighter exhaust emission limits. One solution is the use of mini-four stroke engines. The 25 to 30 cc mini four-stroke engine market is growing and many manufacturers are using this engine design as opposed to alternative two-stroke technology. Carburetor manufacturers therefore have been challenged to provide diaphragm carburetors, both rotary valve carburetors (RVC) and cubic butterfly valve type, for this market. For cost and other reasons such as proven performance and reliability, the approach has been to calibrate diaphragm carburetors of the type used on two-stroke engines of the same size. This process has been mostly successful although performance problems have surfaced related to all position idle stability and acceleration.
In the development of the present invention and while analyzing the reasons for the stability and acceleration problems, it has been discovered that air bubbles are trapped in the metering chamber of the carburetor which impede fuel flow at idle and during acceleration. The problems described above are not as apparent on two-stroke engine applications since two stroke engines consume about three times as much fuel at idle and wide open throttle (WOT) as four-stroke engines. This additional flow increases the velocity of the fuel passing through the fuel circuits which carries (sweeps) vapor and air bubbles from the metering chamber at a rate such that their formation does not have as much affect on engine performance. Since the four-stroke fuel flow rate is much less, the air bubbles are typically not evacuated and are effectively trapped in the metering chamber, agglomerating as by adhering to corners and crevices in the metering chamber. Further, since these engines may tend to run hotter than their two-stroke counterparts, the four-stroke engine carburetors experience an additional heat load which contributes to vaporization of the fuel in the metering chamber. This vaporization creates more bubbles.
Typically, handheld engine diaphragm carburetors feature an easy starting circuit commonly referred to as the Air Purge System. This system is enabled when the user depresses a flexible squeeze bulb which pumps most of the air out of the carburetor""s fuel circuits. A common feature to all carburetors incorporating the Air Purge System is a fuel take off hole inlet for the fuel feeding circuit and a separate check valve inlet for the air purge system, both located in the carburetor metering chamber. Thus, one hole is the inlet from the metering chamber for the air purge routing system, and the other is the inlet from the metering chamber for the fuel circuits supporting the idle and high speed systems. These two holes may be located in different areas within the metering chamber. As a result, the air purge feed hole may not remove enough air/vapor from the metering chamber when operated prior to engine start-up. In this circumstance, upon engine starting, air may be contained in the metering chamber and ingested into the normal idle and high speed fuel circuits. This creates instability at idle and high speed operation since the air creates enleanment when fed into the engine.
Significant effort has been applied to solve the problems of air in the metering chamber. One of the primary prior art solutions identified to date has been to physically position a prior art carburetor on the engine so that the available air purge and fuel circuit feed holes are oriented by carburetor positioning to be generally in the highest position in the metering chamber. This approach has been successful on some engines when the pre-existing feed hole locations happen to match up with the engine manufacturer""s determination of the engine-mounted orientation of the carburetor. However, this approach is not acceptable on other engines due to design and other packaging constraints. Moreover, many times the engine designer mandates an orientation of the carburetor on the engine that results in a customer-mandated standard operation position of the carburetor that has been found to adversely affect the evacuation of the air from the metering chamber, resulting in poor all-position idle stability (stalling) and WOT operation (missing).
Another prior art approach has been the so-called xe2x80x9cMulti-pointxe2x80x9d pickup system. This system features three widely spaced apart take off holes opening into the metering chamber cavity surface and all connected to one single downstream passageway feeding fuel through the air purge check valve to the main jet. A fourth hole opening into the metering chamber communicates with an air purge system. This approach is thus complex, costly and provides only partially satisfactory results in terms of solving the aforementioned problems of gaseous phase accumulation in the metering chamber or incomplete evacuation thereof prior to engine start-up.
Accordingly, among the objects of the present invention are solving one or more of the foregoing problems by providing a new and improved method of reducing gaseous phase presence in the liquid fuel metering of a diaphragm carburetor for an internal combustion engine, such as in the form of excessive air or fuel vapor agglomeration and/or bubble growth due to the same evaporating and/or effervescing from the liquid fuel resident in the metering chamber during engine running, or prior to engine start-up being in the form of an air-filled metering chamber due to carburetor fuel drain-down after engine shut-down.
Another object is to provide an improved method of designing and constructing a diaphragm carburetor of the foregoing character that can be operated to assure that air and vapor bubbles are consumed during the Air Purge starting operation and/or during normal engine operation, and by so consuming this air/vapor volume the engine will exhibit stable all-position idle performance as well as consistent acceleration and provide open throttle (WOT) stability.
A further object is to provide an improved method of the foregoing character that renders the diaphragm carburetor capable of removing the maximum amount of air present in the metering chamber during the actuation of the Air Purge System bulb, and/or that minimizes air bubbles or vapor collecting in the metering chamber during engine idle and WOT operation, and that can also be used with carburetors not equipped with an Air Purge System, and that is applicable to both rotary valve carburetors (RVC) and cubic butterfly type carburetors, and indeed, works well independent of the throttle valve design type.
Still another object is to provide a new and improved diaphragm carburetor designed and constructed in accordance with the foregoing method and operable to produce the improved results achievable by following such method.
In general, and by way of summary description and not by way of limitation, the invention achieves the aforestated objects through special fuel routing in the fuel circuitry of the carburetor. A xe2x80x9cHigh-Point Pick Upxe2x80x9d hole is positioned in the optimal metering chamber location (highest) to assure the maximum evacuation of air during start and running. This optimal location is dependent upon first determining, in advance of carburetor purge system design, the orientation of the carburetor as mounted on the engine in its primary operator usage position, or so-called xe2x80x9cstandard operating positionxe2x80x9d (SOP), and which in turn is determined in the first instance by the engine manufacturer.
Preferably the two typical fuel circuits (air purge and normal idle/high circuits) in diaphragm carburetors are consolidated into one circuit sharing a common sole take-off hole or opening into the metering chamber, and which in turn is so located at the highest point in the metering chamber in the given SOP orientation. This approach assures that the maximum amount of air is removed from the metering chamber during purging. This can be achieved in cubic butterfly valve type carburetors of existing design by the appropriate body machining passage routing of the high-point take off. Many variations are possible. The key is to locate the common take off hole (purge feed and fuel circuit feed) in the highest position within the metering chamber, assuming the primary usage position (SOP orientation) of the engine is a given and known parameter prior to determining such take-off hole location.
Likewise, by utilizing the appropriate body machining passage routing of the high-point take off, the same result can be achieved on an existing typical rotary valve carburetor (RVC). Again, this take off hole can be anywhere in the RVC metering chamber in order to achieve pick up at a predetermined and known SOP metering chamber high-point.